1. Field of the Invention
The present invention relates to a field effect transistor and, more particularly, provides a technology directed toward lowering the on-resistance of a power MOSFET having a high breakdown voltage.
2. Description of the Prior Art
As a lateral power MOSFET in the prior art, a structure shown in FIG. 1, for example, has been known. In FIG. 1, a high impurity concentration n.sup.+ -type Si buried layer 3 is formed between a p-type silicon (abbreviated as "Si" hereinafter) substrate 1 and a p-type Si epitaxial layer 2. An n-type Si drain region 4 is formed in the p-type Si epitaxial layer 2 to be connected to the high impurity concentration n.sup.+ -type Si buried layer 3. P-type Si base regions (channel regions) 5 and a high impurity concentration n.sup.+ -type Si drain region 18 are formed in the n-type Si drain region 4. High impurity concentration n.sup.+ -type Si source regions 6 are formed in the p-type Si base region 5. A gate electrode 11 made of polysilicon is formed on the p type Si base region 5 and a part of the n-type Si drain region 4 via a gate oxide film 7. In addition, a source electrode 12 is formed to be isolated from the gate electrode 11 by a first interlayer film 9. A drain electrode 13 is formed to be isolated from the source electrode 12 by a second interlayer film 10.
In the lateral power MOSFET shown in FIG. 1, if a predetermined potential, e.g., a positive potential is applied to the gate electrode 11 under the condition that a voltage is applied between the drain electrode 13 and the source electrode 12, an n-type inversion layer is formed on a surface of the p-type Si base region 5 immediately below the gate electrode 11 so that a drain current is passed from the drain electrode 13 to the source electrode 12. Conversely, if either 0 V or another predetermined potential, e.g., a negative potential is applied to the gate electrode 11, such n-type inversion layer disappears so that the lateral power MOSFET becomes an OFF state.
However, in the conventional lateral power MOSFET shown in FIG. 1, in order to maintain the breakdown voltage between the drain and the source in the OFF state in excess of a predetermined high value, a concentration of the n-type Si drain region 4 must be reduced and a length between the p type Si base region 5 and the high impurity concentration n.sup.+ -type Si drain region 18 must be made longer. As a result, a current path becomes longer and the on-resistance is increased. That is to say, there is in general a trade-off relation between the breakdown voltage and the on-resistance of the power MOSFET.
As well known in the art, in a so-called abrupt junction wherein it is supposed that a high impurity concentration p.sup.+ type region is connected to an n-type region of relatively low impurity concentration N.sub.d and that a depletion layer is extended only in the n-type region, a breakdown voltage V.sub.B has a relation with an impurity concentration N.sub.d, as expressed by Eq.(1) deduced in compliance with a one-dimensional approximation model. EQU N.sub.d =.epsilon.E.sub.c.sup.2 /(2qV.sub.B) (1)
Where, .epsilon. is a dielectric constant, q is unit charge, and E.sub.c is a critical electric field. A width W of the depletion layer at breakdown can be expressed by EQU W=2V.sub.B /E.sub.c ( 2)
For contrast, a resistance R.sub.d of a semiconductor region having a unit sectional area and a length W can be given by EQU R.sub.d =W/(qN.sub.d .mu..sub.n) (3)
Where, .mu..sub.n is the electron mobility in bulk of respective semiconductor materials.
Furthermore, in the case of abrupt junction, it has been known that, with respect to Si, relations given by Eqs.(4) and (5) can be derived approximately for the impurity concentration N.sub.d and the depletion-layer width W respectively. EQU N.sub.d =2.01.times.10.sup.18 V.sub.B.sup.-4/3 ( 4) EQU W=2.58.times.10.sup.-6 V.sub.B.sup.7/6 ( 5)
In the conventional example of the lateral power MOSFET shown in FIG. 1, if the power MOSFET of 200 V class, for instance, is explained as an example, the impurity concentration of the n-type Si drain region 4 becomes 1.7.times.10.sup.15 cm.sup.-3 based on Eq.(4). In addition, as the distance W between the p type Si base region 5 and the high impurity concentration n.sup.+ -type Si drain region 18, 12.5 .mu.m is needed from Eq.(5). At this time, if the electron mobility .mu..sub.n in the Si bulk is assumed to 1340 cm.sup.2 /V.multidot.s, the drain resistance R.sub.d becomes a large value such as 3.4.times.10.sup.-3 .OMEGA. cm.sup.2 from Eq.(3). In fact, since other resistances such as a contact resistance are added to the on-resistance of the power MOSFET, the drain resistance R.sub.d becomes a larger value. In other words, once the distance W between the base region and the drain region and the impurity concentration N.sub.d in the drain region are defined, a structurally determined breakdown voltage and a correlative value of on-resistance can be derived in the power MOSFET in the prior art, both the breakdown voltage and the on-resistance being insufficient respectively.